Transistors can fail in several ways, however, I think a common failure would be a short between collector and emitter. This failure would pass positive or negative rail directly to the load. This case would generally blow fuses.

Hi Tim,I have this Zenith Troubador record player/radio amp that has a weak left channel which is normally indicative of bad caps.Gave it a complete re-cap and same symptom. Started to check all transistor voltages and one of the left channel output transistor's emitters is 4 volts over spec and the other ones collector is 4 volts over spec.....everything else checks out.

From looking at photo it is looking like a class AB Amp with a positive power supply. The speaker output is through the electrolytic caps which connect to the collectors of the top transistors. If this is an AB Amp then the top transistors are different from bottom, ie NPN VS PNP.

I think you should see roughly 1/2 of the VCC positive voltage at the collectors of the top transistors.

In any case, you should be able to compare right voltages with left and track down the differences. If they don't agree, then the right to left base voltages are probably also disagreeing by a similar amount. This shift in bias is likely being caused by an upstream issue.

Hi Tim,Started to check all transistor voltages and one of the left channel output transistor's emitters is 4 volts over spec and the other ones collector is 4 volts over spec.....everything else checks out.

You normally can't diagnose with just one voltage reading...you have to measure all the voltages and do the math to see where the current is going. If you are not able to link us to a schematic, then post a snip of the area in question.

_________________-Mark "Even if you don't understand Ohm's Law, you are still required to obey it."

[/quote]You normally can't diagnose with just one voltage reading...you have to measure all the voltages and do the math to see where the current is going. If you are not able to link us to a schematic, then post a snip of the area in question.[/quote]

We need more precision in the base to emitter voltages. If the the drop is really 0, the both transistors are shorted. Measure directly from B to E on a setting that gives at least 2 significant figures.

The. B-E drop is normally 0.2 to 0.3 volts for Ge, and higher for Si Your schematic shows 0.1 which suggests they are operating very close to cutoff.

Also, tell us the transistor type number.

_________________-Mark "Even if you don't understand Ohm's Law, you are still required to obey it."

We need more precision in the base to emitter voltages. If the the drop is really 0, the both transistors are shorted. Measure directly from B to E on a setting that gives at least 2 significant figures.The. B-E drop is normally 0.2 to 0.3 volts for Ge, and higher for Si Your schematic shows 0.1 which suggests they are operating very close to cutoff.Also, tell us the transistor type number.

The bias is not correct. The bias is set up by the 4 resistors and the two 3 ohm secondary windings shown to the left of the transistors. Want to be sure those six resistance values are all correct. Also verify the two 0.51 ohm emitter resistors. For emitter resistors you should measure resistance and also verify that they each have the same voltage drop when powered up.

Verify that the top transistor case (collector) is properly isolated from the chassis, the bottom one should be electrically connected to chassis. The white grease should be non-conductive thermal grease.

If all the resistances are correct, then the transistors start to look suspicious. They could be swapped out to see what her the problem can be made to move to opposite channel.

That circuit is self-balancing, but only if the transistors are reasonably well matched. If the midpoint goes high, it decreases the drive to the upper transistor, and INcreases the drive to the lower. If the the midpoint is low, the opposite happens.The actual mid-point voltage can be calculated by replacing the transistors and the emitter resistors with resistors whose value is 0.5 ohms times the DC beta of the transistors (90 nominal--from the NTE-121** data sheet) Solving the resulting circuit shows that the mid-point voltage depends on the matching of the transistors. If they are not matched, the circuit still works, but will clip at a lower volume level.

Based on the measured voltages, either one transistor is leaking, or they are significantly mismatched.

I forget if this channel works to some degree, or is totally dead.

**google says this is the replacement for the number in your picture.

_________________-Mark "Even if you don't understand Ohm's Law, you are still required to obey it."

The 'higher than balanced' voltage of 25V at the center node should be trying to reduce conduction on the top transistor and increase on the bottom, in effect trying to balance out as you stated. This all assumes that the resistor values are appropriate.

Also just noticed that the feedback loop is DC coupled, so if the node voltage is not at the designed 20V, or so, then the DC offset will also have some biasing effect on the driver Amp. This could be affecting gain and volume. Maybe not significant.

If you can remove the transistor without risk or major effort, the diode tester on a multimeter is a quick way to check for faults. Just swap probes between base and collector and one direction will show open circuit. Same for base collector.

There are 2 identical circuits stacked together---sometimes called a "totem-pole" configuration. Each half is a current source controlled by the voltage applied to the base of the transistor.

I said something misleading earlier---this circuit is actually relative INsensitive to changes in the Beta of the transistor.Simplified analysis follows:

First, the output current is controlled by the voltage applied to the base of the transistor. As an approximation, we can assume a very high Beta, and no base-emitter drop, in which case the current is simply the applied voltage divided by 0.51 ohms.

On the base side, we'll start with the divider equivalent circuit, expressed as a negative offset from the +42 (or from +21 when applied to the lower half)open-circuit voltage: ( 5.6 / 685.6 ) * 21 = 0.17 voltsource resistance: 5.6 || 680, which is ~ 5.6----then add the 3 ohms for a total of 8.6 ohms

To get the base voltage, we next approximate the base load resistance by simply multiplying the emitter resistor by Beta: let's use 100, giving a reflected load of 5.1 ohms.

Finally, find the base voltage = ( 5.1 / 13.7 ) * 0.17 = 0.063. Again assuming no B-E drop, we get the current by dividing this voltage by the emitter resistance:Ic = 0.063 / 0.51 = 0.124 ====> 124mA (This is the approximate idling current for each 1/2 of the circuit.

To get the AC gain, recognize that the applied voltage is only what is developed in the secondary of the driver transformer. A slightly different analysis is needed to find the resulting collector current.

Getting back to the issue with the circuit:From the above we can see that the quiescent output voltage will be relatively insensitive to Beta, unless of course one of the transistors fails to ZERO beta. I think the 25 volt reading at the midpoint points to the upper transistor being leaky.

None of this changes the suggested diagnosis of simply removing the transistors and testing them. (Checking resistors first would be a normal 1st step, but they probably cannot be correctly tested in-circuit. This may favor testing transistors first)

_________________-Mark "Even if you don't understand Ohm's Law, you are still required to obey it."

There are 2 identical circuits stacked together---sometimes called a "totem-pole" configuration. Each half is a current source controlled by the voltage applied to the base of the transistor.

I said something misleading earlier---this circuit is actually relative INsensitive to changes in the Beta of the transistor.Simplified analysis follows:

First, the output current is controlled by the voltage applied to the base of the transistor. As an approximation, we can assume a very high Beta, and no base-emitter drop, in which case the current is simply the applied voltage divided by 0.51 ohms.

On the base side, we'll start with the divider equivalent circuit, expressed as a negative offset from the +42 (or from +21 when applied to the lower half)open-circuit voltage: ( 5.6 / 685.6 ) * 21 = 0.17 voltsource resistance: 5.6 || 680, which is ~ 5.6----then add the 3 ohms for a total of 8.6 ohms

To get the base voltage, we next approximate the base load resistance by simply multiplying the emitter resistor by Beta: let's use 100, giving a reflected load of 5.1 ohms.

Finally, find the base voltage = ( 5.1 / 13.7 ) * 0.17 = 0.063. Again assuming no B-E drop, we get the current by dividing this voltage by the emitter resistance:Ic = 0.063 / 0.51 = 0.124 ====> 124mA (This is the approximate idling current for each 1/2 of the circuit.

To get the AC gain, recognize that the applied voltage is only what is developed in the secondary of the driver transformer. A slightly different analysis is needed to find the resulting collector current.

Getting back to the issue with the circuit:From the above we can see that the quiescent output voltage will be relatively insensitive to Beta, unless of course one of the transistors fails to ZERO beta. I think the 25 volt reading at the midpoint points to the upper transistor being leaky.

None of this changes the suggested diagnosis of simply removing the transistors and testing them. (Checking resistors first would be a normal 1st step, but they probably cannot be correctly tested in-circuit. This may favor testing transistors first)

Possibly leaking capacitors could alter transistor voltage. DC electrons are free to flow through the dielectric. If a radio is really getting on in years it may be an idea to change the capacitors regardless. In fact, the audio improves a heck of a lot. And when you charge the old caps and discharge across a speaker, the pop is hardly audible. If after all that the transistors seem off spec, be gentle on removal. I tend to wiggle out a leg at a time with quick heat applied. Too much heat and tracks may peel. I removed before AF117s and found leaks to screen lead and case.

If feedback is removed, channel gets louder. If feedback is theproblem, then the weak channel may be the good channel.

When it is decided which channel is bad, remove output transistorsand check with transistor tester, and/or multimeter., If it is determined that output transistors , both R & L channels, test OK,then on the bad channel , with output transistors out, connect a test speakeracross each 3 ohm secondary of driver transformer, and make surethe quieter signal you hear, is the same for both windings.

Right channel / left channel A B this test to determine if the drivertransistor stage is root of problem.

Transistors in an amplifier like this that test OK for shorts,and beta, compensate themselves because of how the circuitis designed.

A cranky boss might have growled at a technician who decidedto go all-academic on a record player. Just fix the damn thing !

_________________deVE7ASO VE7ZSOAmateur Radio Literacy Club. May we help you read better.Steve Dowve7aso@rac.ca